Mobile data transmission and data services are constantly making progress, wherein such services provide various communication services, such as voice, video, packet data, messaging, broadcast, etc. Such systems may be systems for multiple-access, which are capable of supporting multiple users by sharing the available system resources. In recent years, Long Term Evolution LTE™ and Long Term Evolution Advanced LTE™-A have been specified.
Further, machine-type Communications MTC is a market that is likely to continue expanding in the future. Thereby, more and more MTC devices are targeting low-end (i.e. low cost, low data rate) applications that can be handled adequately by GSM/GPRS.
According to 3GPP RAN#57 in document [1], one challenge of low cost MTC is to enhance the coverage of low cost MTC user equipments UEs with very low data rate. Thereby, according to [1], it shall be ensured that service coverage is not worse than GSM/GPRS, at least comparable and preferably improved beyond what is possible for providing MTC services over GPRS/GSM today (assuming deployment in the same spectrum bands).
A 20 dB improvement in coverage in comparison to defined LTE cell coverage footprint as engineered for “normal LTE UEs” should be targeted apply for low-cost MTC UEs, using very low rate traffic with relaxed latency (e.g. size of the order of 100 bytes/message in uplink UL and 20 bytes/message in downlink DL, and allowing latency of up to 10 seconds for DL and up to 1 hour in uplink, i.e. not voice).
The requirement on such coverage improvement of 20 dB comes from the deployment of the low cost MTC UEs, such as, according to document [2] (RP-120715), meters tend to be installed deep inside buildings (basement etc.)
According to document [3] (TR 36.888), there is the section 5.2.1 on Methodology for performance evaluation where the link budget is used as the method for coverage analysis, and results on MCL calculation for normal FDD and normal TDD are given in Table 5.2.1.2-2 and Table 5.2.1.2-3 respectively and the table for FDD is as following.
The following table 1 shows MCL calculation for normal LTE frequency division duplex FDD according to [3]. Thereby, it is to be noted that eNB (base station) is assumed with 2 Tx (transmitters) and 2 Rx (receivers) in FDD systems.
Physical channel namePUCCH (1a)PRACHPUSCHPDSCHPBCHSCHPDCCH (1A)Data rate(kbps)2020Transmitter(0) Max Tx power (dBm)23232346464646(1) Actual Tx power (dBm)23.023.023.032.036.836.842.8Receiver(2) Thermal noise density−174−174−174−174−174−174−174(dBm/Hz)(3) Receiver noise figure (dB)5559999(4) Interference margin (dB)0000000(5) Occupied channel bandwidth (Hz)1800001080000360000360000108000010800004320000(6) Effective noise power =−116.4−108.7−113.4−109.4−104.7−104.7−98.6(2) + (3) + (4) + 10 log((5)) (dBm)(7) Required SINR (dB)−7.8−10.0−4.3−4.0−7.5−7.8−4.7(8) Receiver sensitivity =−124.24−118.7−117.7−113.4−112.2−112.5−103.34(6) + (7) (dBm)(9) MCL = (1) − (8) (dB)147.2141.7140.7145.4149.0149.3146.1
According to document [1], the 20 dB improvement in coverage is in comparison to the defined LTE coverage footprint as engineered for “normal UE”, whereby the minimum MCL in the above mentioned tables can be used as reference for comparison to get the 20 dB improvement. Based on the results in above table, the required improvement for each channel as shown can be obtained as is shown in table 2 below.
Table 2 shows MCL calculation for normal LTE FDD.
Physical channel namePUCCH (1a)PRACHPUSCHPDSCHPBCHSCHPDCCH (1A)(9) MCL = (1) − (8) (dB)147.2141.7140.7145.4149.0149.3146.1Required improvement (dB)13.5192015.311.711.414.6140.7 + 20 − MCL of each channel
It is obvious that with the 20 dB improvement target for the worst channel (Physical Uplink Shared Channel PUSCH in this FDD case), all the DL/UL channels have to be enhanced, including synchronization channel SCH, broadcast channel BCH and Physical Downlink Control Channel PDCCH channels.
As becomes apparent from table 2 above, about 14.6 dB need to be improved for PDCCH channel, however, this is the worst case. In particular, it can be expected that not all low cost MTC UEs are located in deep basement, then, depending on different deployments, the required coverage improvement can vary. Thus, from resource efficiency and power saving points of view it is desirable to enable the provision of various degree of coverage improvement.
In studies on UL coverage enhancement according to documents [4] and [5], transmission timing interval TTI bundling enhancements are discussed. In these studies, it had been proposed that there can be different TTI bundling length to provide various gains. However, TTI bundling currently is only applied to LTE UL transmission, if such TTI bundling or repetition is to be applied to DL control channels, there are at least following problems to be solved:                Position of the 1st transmission;        Number of repetition/bundling length; and        Whether to use repletion/TTI bundling in DL control transmission to one UE.        
However, these are specific problems for DL control channel enhancement. Taken the position of 1st transmission as example, there is no such ambiguity in data transmission, since data transmission are scheduled via grant which implicitly or explicitly determines the time for 1st transmission.
However for DL control channels, there is no such grant, then UE will not know when the 1st transmission of DL control is sent. Similarly, for data transmission with TTI bundling or repletion, the repetition length can also be easily know from the grant, then there is no ambiguity at UE side on which subframes to combine during detection, while for DL control repletion, this is no such knowledge at UE side in advance. Both problems can be solved via blind detection, however, without any knowledge on start position and repetition length, a huge number of blind detections will be required, which is not desired.
As for the 3rd problem, it occurs mainly at eNB side and will affect the resource efficiency, e.g., if eNB always assume worst coverage for the UE and always use longest repetition in transmission, the coverage can be guaranteed, however, much resource is used unnecessarily, and results in low spectrum efficiency and low throughput of the network.
Hence, there is the need to avoid/reduce the problems mentioned above in downlink control channel repetition.